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Four days ago I discovered something interesting which I've been trying to rule out ever since. Perhaps someone else can help with this, because it's putting up a strong fight. The key idea here is that if the path of light is deflected slightly by the existence of a massive object nearby (through any mechanism, including the one in General Relativity), it looks as if it could lead to an imbalance of forces in matter which will result in a force being generated that would accelerate it towards the massive object. In the case of GR, this would accelerate matter off the geodesic which it should be following.I happened upon this mechanism while thinking about length contraction in LET (Lorentz Ether Theory) and what might happen in a gravity well, so the best place to start is there. Start out by imagining a room in which all walls, floor and ceiling are square. There is a light in the middle which illuminates all these surfaces equally, though each square surface will be a little brighter in the middle and darker in the corners due to the distance the light has to travel before it reaches them - the further it goes, the more it will spread out and the dimmer it will be when it arrives.If the room is moving along at high speed, the light will have further to go before it can reach the leading wall, but it will also have less far to go to reach the trailing wall, so you would expect the leading wall to get dimmer with higher speed of the room and the trailing wall to get brighter, but that doesn't happen - the way the light is emitted is affected by the speed of travel of the lamp, so more light is concentrated forwards and less backwards, helping to reduce the difference in brightness between these two walls, but they will both be dimmer unless the room contracts in the direction of travel.[This concentration of light forwards and reduction backwards can be understood mechanistically if you imagine sending the light out sideways and reflecting it off a flat mirror - the movement of the mirror will make it act as if it is curved. If you think things through with lenses, you'll find similar effects which ultimately show you why light has to be concentrated forwards and reduced backwards whenever a light source moves.]But why should the room contract? Well, the forces holding atoms apart are also transmitted at the speed of light and they spread out in the same way, weakening over distance. This means that the atoms will naturally settle closer together in the direction of travel whenever they find themselves too far apart. By the time the forces have pulled all the atoms into the right places, the illumination will be equal again on both the leading and trailing walls, just as if the room was stationary.Now we can turn to a situation involving gravity where we have a very slight curvature of the paths followed by photons (caused by the nearby presence of a massive object like a star or planet), but these deviations will affect force carriers in the same way as they do with light. The result will be an extra spreading out of the forces in the direction away from the massive object and a reduced spreading towards the massive object. With light, this means that the wall furthest from the massive object will be dimmer than the wall nearest to the massive object, and it will be the same with the forces being applied. The result of this is that the wall furthest from the massive object will be pulled inwards towards the centre of the room because it's sitting further out from where the balance of force requires it to be, but the wall nearest the massive object will be pushed outwards away from the centre of the room because it's sitting to close in from where the balance of forces requires it to be. Both walls are being pushed by this towards the massive object.It wouldn't work quite like that, of course, because rather than a room we should be thinking about forces moving about within the atom nucleus, or even within electrons and quarks. The distances travelled by these force carriers would be very short and would have little opportunity to bend, but the effect would still build up rapidly over time as these forces are being applied continually, so the imbalance would be absolutely real and must generate a force towards the massive object.So, can anyone shoot this down?
I have worked it through and see your point exactly. Isn't this just a description of gravity? What makes you think this wouldn't follow a geodesic?
It is special relativity where no forces act. General relativity is different.http://csep10.phys.utk.edu/astr162/lect/cosmology/gravity.html
David have a look at this video and in the still frame before the slow motion look at the compression of the slinky at the bottom compared to the top. Think of this in relation to you mass deviation.//www.youtube.com/watch?v=rCw5JXD18y4Now how would this change with altitude? If at all.
Quote from: jeffreyH on 05/03/2015 18:15:19It is special relativity where no forces act. General relativity is different.http://csep10.phys.utk.edu/astr162/lect/cosmology/gravity.htmlBut in GR, gravity is not a force - time and space are warped in such a way that things can just follow straight lines and make it look as if there's a force acting when there isn't one. It may be that things are so warped by this though that the room in my thought experiment is distorted in such a way that there is no imbalance of forces, but I'm still trying to work out whether that's possible.Quote from: jeffreyH on 05/03/2015 18:35:20David have a look at this video and in the still frame before the slow motion look at the compression of the slinky at the bottom compared to the top. Think of this in relation to you mass deviation.//www.youtube.com/watch?v=rCw5JXD18y4Now how would this change with altitude? If at all.Nice video - I was wanting to find one like that, but I'm not sure how it relates to this. You aren't seeing compression there, but the opposite, and the stretching at any point is just a measure of the mass further down.[Someone should do a video of a slinky being dropped alongside a ball (dropped from the same height as the top of the slinky, and released at the same moment). I'd like to see if the top of the slinky would descend much more quickly than the ball.]
But in GR, gravity is not a force - time and space are warped in such a way that things can just follow straight lines and make it look as if there's a force acting when there isn't one.
Quote from: David CooperBut in GR, gravity is not a force - time and space are warped in such a way that things can just follow straight lines and make it look as if there's a force acting when there isn't one.That's a common misconception, David.
To see how to obtain the expression for the gravitational force in GR please see:http://home.comcast.net/~peter.m.brown/gr/grav_force.htm
People think that the gravitational force is really a manifestation of curved spacetime.
In the first place that has to do only with tidal forces and not gravitational forces in general. And spacetime curvature is quite literally just another name for tidal forces.
But you can certainly have gravitational forces in the absence of spacetime curvature. I've calculated it myself here:http://home.comcast.net/~peter.m.brown/gr/uniform_force.htmIf you'll notice Eq. (12) you'll see that it's identical to the Newtonian expression.
The only think that GR introduces is the notion that all gravitational forces are inertial forces and that in GR inertial forces are "real" whereas in Newtonian mechanics they're mostly considered to be a manifestation of the coordinate system being a non-inertial one.